What is the difference between certain and sure?
I do not understand the grammatical differences between them.
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It depends on the context in which both words are used. Generally, there is little to no difference. However, a person may use certain to add more power to his statement when he is completely positive. For example:
I am certain that Costa Rica beat Italy in the World Cup match today.
A greater emphasis is added to one's sentence when using the word certain. On the other hand, when using sure it does not always sound confident, and you can tell that in the way it is pronounced most of the time. For example:
I'm sure Costa Rica beat Italy in the World Cup match today.
I'm sure I left my mobile at home.
I'm certain I left my mobile at home.
With certain you completely believe that you left your mobile at home. I hope this has answered your question.
There is no grammatical difference. Both are adjectives and the meaning is the same. But there is a difference in frequency. In colloquial language "sure" is the word used, whereas "certain" is a variant that is less frequent in spoken language. In written language "certain" can be used as a means of variation.
Oald says in entry sure that sure and certain are synonyms.
Sure can be an adjective or an adverb. Certain can be an adjective or a pronoun or determiner. Related adverbs surely and certainly also exist, as do derived nouns sureness and certainty, which have different meanings.
It is almost unbelievable that some people here said that the two mean the same.
To be sure, in their adjectival uses they are often interchangeable. (Mind you, I could not have written "To be certain" at the start of the last sentence. "To be sure" means "it is certainly the case that ...", whereas "to be certain" is used with the sense "in order to be certain", or as an infinitival complement in expressions like "I wanted to be certain". "To be sure" will work in those, too, but it has an extra sense as well.)
In these and many other examples from Cambridge, the two words are interchangeable:
Are you absolutely certain (that) you gave them the right number?
I feel certain (that) you're doing the right thing.
I'm sure (that) I left my keys on the table.
I feel absolutely sure (that) you've made the right decision.
It now seems sure (that) the election will result in another victory for the government.
With the following examples, I am less sure of the interchangeability. The expression "not sure" seems to connote a greater degree of doubt than "not certain". While "not certain" tends to mean that you have a pretty good idea but are not yet certain, "not sure" is perhaps more likely to be a euphemism for not really knowing or not wanting to say:
"What's wrong with him?" "I'm not really sure."
Shaun isn't sure whether/if he'll be able to come to the party or not.
Is there anything you're not sure of/about?
In colloquial English, especially in AmE, "sure" acts as an adverb meaning "certainly", "indeed", etc. It can also be used as an interjection, as another way of saying "yes".
"Do you want to come swimming with us?" "Sure."
I sure am hungry (= I am very hungry).
("Certain" is impossible here, although "certainly" could be used in both sentences, but might sound too formal to Americans.)
In the phrase "to be sure of oneself" (to feel confident), "sure" is used:
She’s much more sure of herself since she started work.
In impersonal expressions of likelihood, "sure" and "certain" can both be used:
After all his hard work, he's certain to pass his exams.
However, where the certainty is qualified as "virtually", "almost", the word "certain" is much more likely to be chosen:
The team looks almost certain to win the match.
It is virtually certain (that) she will win the gold medal.
There are also certain expressions such as "face certain death" or "face certain defeat" where "sure" is virtually unknown.
As a determner and pronoun, "certain" is used in expressions such as "a certain Mr Smith", "certain issues" (= "some issues"), "certain of the soldiers", etc - where "sure" is impossible.
"Certainly" as an adverb refers to things that are certain, and can also be a way of agreeing with a statement:
She certainly had a friend called Mark, but I don't know whether he was her boyfriend.
"This is rather a difficult question." "Yes, it's certainly not easy."
"Do you think more money should be given to education?" "Certainly!"
In AmE, "surely" is often used to mean the same thing as "certainly"; in BrE it rarely is. But in BrE (and sometimes AmE), "surely" is used with a different set of meanings, as a way of strongly suggesting something without actually meaning that it's certain:
his latest injury must surely mean that her tennis career is now at an end.
He's running so well, surely he'll take the gold.
Greater international stability can surely only be to the good.
It's also used to express surprise or indignance or to try to dissuade someone:
Surely you don't expect me to believe that?
Surely you're not going out on a night like this?
Finally, the noun "sureness" is used primarily in expressions such as "sureness of touch" (confidence), whereas "certainty" refers to the state of being certain or sure about something.